PSI - Issue 2_A

F. Tolea et al. / Procedia Structural Integrity 2 (2016) 1473–1480 Author name / Structural Integrity Procedia 00 (2016) 000–000

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the ribbons subjected to successive “in situ” thermal cycles up to the temperature at which the MT was no more observed by DSC, are shown in Fig. 3a, for Ga25, Ga26 and Ga28 studied samples. The first observation is the missing of the secondary  phase in the pattern recorded on the AQ ribbons of the all alloys. In concordance with the DSC results, Ga25 and Ga28 as prepared ribbons are, at room temperature, in martensite phase and the main reflections may be indexed as a modulated 7M type orthorhombic structure. The as prepared state for Ga26 is characterized by the coexistence of the martensite phase with the same orthorhombic structure and the austenite phase with disordered B2 cubic structure. It is to note that the XRD patterns were recorded on the ribbons free sides and not on the sides in direct contact to the wheel surface, during the melt spinning process. However, due to the different cooling velocity between the contact side and the free side a high texture is generally (usually) developed on ribbons [Okumura et al. (2010); Wang et al. (2013)]. For Ga26 the texture is marked by the enhanced intensity of austenite [200] and [400] reflections while for the ribbons in martensite state, the [004] reflection is enhanced.

Fig.3 a) The X-ray diffraction patterns registered at room temperature for Ga26 and Ga28 alloys on AQ as well as on the ribbons after final DSC scans. b) The XRD patterns at room temperature for Ni50Mn25 on AQ ribbons and Ni57Mn22 alloys on AQ and after final DSC scans.

Almost identical XRD spectra were obtained on the ribbons subjected to in situ repeated thermal cycles. The reflections peaks are indexed as belonging to the  phase and to austenite with B2 structure. However, the large broadening of the peaks suggests the reduction of the crystallite size. This may be due to the intra-granular  phase precipitates reducing the size of the transformable phase in the grains, as also observed in the case of bulk alloys [Masdeu et al. (2008)]. As is revealed in Fig.3b, Ni50Mn25 sample show reflection peaks of L2 1 austenite structure. Ni57Mn22 sample is at RT in the martensite phase with 7M tetragonal modulated structure. After DSC “in situ” progressive treatments, γ phase precipitated Ni57Mn22 and the peaks become larger, so repeated DSC scans produce an important decrease of the crystallite size. Specific morphologies as evidenced by SEM on AQ ribbons as well as on thermally treated ribbons are shown in Fig. 4. At RT martensite and austenite states coexists on the Ga26 AQ ribbons. Accordingly, the microstructure shows variants of the twinned martensite (Fig 4a). The textured structure observed by XRD is now proven by the SEM images on the fractured cross section of the as prepared Ga26 ribbons (Fig 4d insert). Due to the gradient of cooling rate across the ribbon, the grains placed on the contact side (bottom of the inset figure) are small and equiaxial whereas columnar grains grow upward to the free surface. A dramatic change induced by the repeated thermal treatments on the microstructure is observed on Ga26. For this alloy, the last observed reverse transformation is well below room temperature so that Fig. 4b shows the austenite microstructure in the last stage of the MT degradation, the high density of cracks gives an aspect of porous material that was also suggested by the brittleness of the ribbons and which could be seen also in fracture (Fig.4e). SEM images on Ni57Mn22 (Fig. 4c) emphasize the twinned morphologies of the martensite. After TT (Fig. 4f) voids and cracks give the evidence for the degradation of the microstructure. 3.3 Magnetic measurements This section presents magnetic thermo-measurements on the Ni-Fe-Ga alloys. With respect to the magnetic